1. Field of the Invention
This invention relates to a plasma CVD (chemical vapor deposition) apparatus suitable for the manufacture of thin films of large areas which are used in various electronic devices, such as amorphous silicon solar cells, thin film semiconductors, optical censors, and protective films for semiconductors.
2. Description of the Related Art
With reference to FIG. 7, we shall describe a conventional plasma CVD apparatus which has been used for the manufacture of thin amorphous silicon films with large areas. This technology is known as disclosed, for example, in Japanese patent application No. 106314/1986 (61-106314).
Electrodes 2, 3 are disposed in parallel with each other in a reaction container 1 for generating glow discharge plasma. Electric power with a commercial frequency of, for example, 60 Hz is supplied to these electrodes 2, 3 from a low-frequency power supply 4. A DC or high-frequency power supply can also be used. A coil 5 is wound around the reaction container 1, and AC power is supplied from an AC power supply 6. A gas mixture of, for example, monosilane and argon is supplied to the reaction container 1 from a cylinder (not shown) via a reaction gas introduction pipe 7. The gas in the reaction container 1 is exhausted through an exhaust pipe 8 by a vacuum pump 9. A substrate 10 is located outside the discharge space formed by the electrodes 2, 3 and supported in the direction perpendicular to the faces of the electrodes 2, 3 by a suitable means.
Using this apparatus, a thin film can be manufactured in the following manner. The vacuum pump 9 is driven to remove gas from the reaction container 1. Through reaction gas introduction pipe 7, a gas mixture of, for example, monosilane and argon is supplied. The pressure inside the reaction container 1 is maintained at 0.05 to 0.5 Torr, and electric voltage is applied to the electrodes 2, 3 from the low-frequency power supply 4. Glow discharge plasma is generated. An AC voltage of, for example, 100 Hz is applied to the coil 5 to generate a magnetic field B in the direction perpendicular to the electric field E generated between the electrodes 2 and 3. The magnetic flux density in this magnetic field can be about 10 gausses.
Of the gas supplied from the reaction gas introduction pipe 7, monosilane gas is decomposed by the glow discharge plasma generated between the electrodes 2 and 3. As a result, silicon (Si) radicals occur and attach to the surface of the substrate 10 to form a thin film.
Charged particles, such as argon ions, take the so-called E.multidot.B drift motion because of the Coulombic force F.sub.1 =q.multidot.E and the Lorentz force F.sub.2 =q(V.multidot.B), where V is the velocity of a charged particle. The charged particles are given an initial velocity by this E.multidot.B drift and fly in the direction perpendicular to the electrodes 2, 3 toward the substrate 10. However, in the discharge space where the effect of the electric field between the electrodes 2 and 3 is small, the charged particles fly following a Larmor trajectory because of the cyclotron motion due to the magnetic field B generated by the coil 5. Therefore, the charged particles, such as argon ions, rarely hit the substrate 10 directly.
The silicon (Si) radicals, which are electrically neutral, are not influenced by the magnetic field B and divert from the above trajectory of the charged particles to reach the substrate 10 and form a thin amorphous film on the surface thereof. Because the Si radicals collide with the charged particles flying along the Larmor trajectory, the thin amorphous film is formed not only in front of the electrodes 2, 3, but also in areas to the left and the right thereof. Furthermore, because the magnetic field B is varied by the AC power supply 6, the thin amorphous film can be formed on the surface of the substrate 10 uniformly. Also, because the electrodes 2, 3 can be long as long as they fit inside the reaction container 1, even if the substrate 10 is long, the thin amorphous film can be formed uniformly on its surface.
According to the conventional apparatus described above, a film can easily be formed on a large area by generating a magnetic field B in the direction perpendicular to the discharge electric field E between the electrodes generating glow discharge plasma. However, this apparatus has the following problems.
(1) When a film of large area is formed, the electrodes need to be long. In order to generate stable plasma using long electrodes, the frequency of the power supply should be as small as possible. A power supply with a frequency of several 10 to several 100 Hz is therefore used. However, under the conditions in which the frequency becomes small and the ion transport during a half period exceeds the distance between the electrodes, secondary electrons discharged from the negative electrode (cathode) due to collisions between the ions play an essential role in maintaining the plasma in the same way as in DC discharge. Therefore, if a film forms on the electrodes and the electrodes become insulated by the film, discharge does not take place in the insulated portion. In this case, the electrode surfaces have to be kept always clean. Therefore, troublesome operations, such as exchanging and cleaning the electrodes very often, are required, and it is a reason for higher costs.
(2) If a high-frequency plasma source of, for example, 13.56 MHz is used in order to alleviate the above disadvantage (1), the secondary electrons discharged from the electrode become inessential in the maintenance of the discharge. Then, even if there exists some insulator, such as a film, on the electrode, glow discharge still forms between the electrodes. However, if the electrodes used are long, because of the skin effect of high frequency, most of the electric current flows in the surface (about 0.01 mm) and thus the electric resistance increases. For example, if the length of the electrode is 1 m or more, some potential distribution appears on the electrodes and uniform plasma does not result. If we consider this in terms of a distribution constant circuit, it can be shown as in FIG. 8. In FIG. 8, x indicates the distance in the length direction of the electrode. If the resistance R per unit length of the electrode is so large that it cannot be ignored compared with the impedance Z.sub.1, Z.sub.2, . . . , Z.sub.n of the discharge portion, a potential distribution appears in the electrode. Therefore, when a high-frequency power supply is used, it is very difficult and has not been possible in practical applications to form a film having a large area.
(3) According to the methods (1) and (2) above, when a thin amorphous silicon film of 50 cm.times.50 cm or larger is produced, it has been extremely difficult to keep the distribution of film thickness within .+-.10% and maintain the speed of film formation at 1 .ANG./sec or more.